1. Field of the Invention
The present invention relates to an electromagnetic flow meter that measures the flow rate of a fluid to be measured that is flowing through a measurement tube, and more particularly relates to a capacitative electromagnetic flow meter.
2. Description of the Related Art
There are two types of electromagnetic flow meter, namely, the liquid-contacting electrode type electromagnetic flow meter, in which an electrode is directly in contact with the fluid to be measured and the electro motive force (hereinbelow called the e.m.f.) generated in the fluid to be measured is directly detected, and the non-liquid-contacting electrode type electromagnetic flow meter (hereinbelow termed a capacitative electromagnetic flow meter), in which the electrode is not directly in contact with the fluid to be measured and the e.m.f. generated in the fluid be measured is detected through the electrostatic capacitance between the fluid to be measured and the electrodes.
Furthermore, an electromagnetic flow meter is required to obtain a stable flow rate signal from which the effect of noise has been removed. However, this noise has various causes, so a large number of types of electromagnetic flow meter exist, depending on the different means used to effect such removal (see for example Laid-open Japanese Patent Publication No. H. 8-304132 (referred to as Patent Reference 1)).
Various types of anti-noise measures are known that have been subsequently developed to improve the capacitative electromagnetic flow meter disclosed in this Patent Reference 1 (for example Laid-open Japanese Patent Publication No. 2001-116598 (referred to as Patent Reference 2)). The construction and action of these will be described with reference to FIG. 1 to FIG. 3.
First of all, the construction thereof will be described with reference to FIG. 1. As shown in this Figure, this capacitative electromagnetic flow meter comprises a detection unit 10 and a signal processing unit 11 that is used to find the flow rate from the detected signal e detected by the detection unit 10.
The detection unit 10 applies magnetic flux by forming a return magnetic circuit, not shown, with respect to the fluid 2 to be measured, by passing an exciting current iF from an exciting circuit 8 to exciting coils 3A, 3B wound on magnetic poles 7 arranged facing the outer wall of the measurement tube 1, made of an insulating substance, through which the fluid 2 to be measured flows.
Amplifiers 6A, 6B are used to amplify the e.m.f. proportional to the flow rate of the fluid 2 to be measured, mentioned above, through the electrostatic capacitance between a pair of face electrodes 4A, 4B that are arranged facing the outer wall of the tube 1 where measurement is conducted in a direction orthogonal to the direction of this magnetic flux and guard electrodes 5A, 5B and the measurement tube 1 and the respective face electrodes 4A, 4B referred to above, and between the face electrodes 4A, 4B and guard electrodes 5A, 5B arranged so as to cover these face electrodes 4A, 4B and, in addition a difference amplifier (or differential amplifier) 6C amplifies the difference eAB of the respective signals from the amplifiers 6A, 6B, thereby performing detection of the detection signal e.
Next, flow rate measurement is conducted by passing this detection signal e to a signal processing unit 11, which samples positions other than the region of rise of the detection signal e (termed differentiation noise).
In this system, the impedance between the face electrodes 4A, 4B and the fluid 2 to be measured is extremely high, so various types of anti-noise measures are provided in the detection unit 10.
One of these anti-noise measures is in respect of noise that is induced between the face electrodes 4A, 4B. This anti-noise measure involves maintaining the guard electrodes 5A, 5B at the same potential as the face electrodes 4A, 4B and removing noise induced in the same phase between the face electrodes 4A and 4B by performing amplification by the difference amplifier 6C after impedance conversion using the amplifiers 6A, 6B.
Also, in the magnetic flux circuit between the guard electrodes 5A, 5B and the exciting coils 3A, 3B, damping foil 7A, 7B, to be later described, may be arranged.
In addition, grounding of such a detection unit 10 is achieved by connecting to ground G by connecting the earth E of a metal pipe casing liquidly connected with the periphery, not shown of the measurement tube 1 and a common potential earth C of the circuit.
Noise, called differentiation noise, as described above, is superimposed on the detection signal e of a capacitative electromagnetic flow meter constructed in this way.
This noise is induced in the detection loop formed between the two face electrodes 4A, 4B and the amplifier 6A, 6B by induction due to electromagnetic coupling with the exciting magnetic flux and the difference of the potential fluctuations between the two ground points G and the respective face electrodes 4A, 4B that occur when the exciting magnetic flux fluctuates is superimposed on the rising portion of the detection signal e as noise.
The details of this will be described using FIG. 2A, FIG. 2B, FIG. 2C, FIG. 2D and FIG. 2E. When a square wave exciting current iF as shown in FIG. 2A flows in the exciting coils 3A, 3B, the rising portion of the exciting magnetic flux "PHgr" shows a waveform whose characteristic is somewhat blunted by the response time constant of the diamagnetic field action in the exciting magnetic circuit, as shown in FIG. 2C, by the eddy current iE generated in the exciting magnetic flux path, as shown in FIG. 2B.
Due to these changes of the exciting magnetic flux "PHgr", noise in differential form i.e. differentiation noise is superimposed on the rising portion of the detection signal e as described above, as in the portion Nd in FIG. 2D.
It is therefore necessary for the construction within the detector 10 to be set up and arranged such that the eddy current iE generated in the exciting magnetic circuit is kept to a minimum.
Also, in order to detect the stable component of the flow rate signal, as shown in FIG. 2E, the flow rate is found by sampling with the timing of a sampling signal SP at which the value of the differentiation noise has become small.
Apart from the differentiation noise described above, low-frequency noise, called xe2x80x9cfluid noisexe2x80x9d is superimposed on the detection signal e. The mechanism of generation of this fluid noise is inferred to be that low-frequency potential fluctuations are produced in the fluid 2 to be measured, due to irregular movements of the ions that are transported by the fluid 2 to be measured. Such fluid noise increases when the flow rate of the fluid 2 to be measured becomes faster.
In order to separate this fluid noise and the e.m.f. that is proportional to the flow rate, the frequency of the exciting current is made higher than the frequency of the commercial supply (or commercial frequency) and the exciting circuit is set such that the flux waveform settles down in a short time.
However, since the inductance of the exciting coils 3A, 3B has a characteristic having a resonant point in the high frequency region in the vicinity of 50 kHz, the phenomenon of oscillation of the exciting current iF as shown in FIG. 3 still occurs even though the exciting current iF is controlled with high frequency.
For this reason, thin conductive sheets called damping foils 7A, 7B are provided between the exciting coils 3A, 3B and the guard electrodes 5A, 5B in order to eliminate the resonant point of the oscillation.
As described above, in a conventional capacitative electromagnetic flow meter, the excitation frequency of the exciting current is made higher than the commercially supplied frequency in order to avoid the effect of fluid noise and damping foil is provided in the flux path in order to suppress oscillation of the exciting current produced by this raising of the exciting frequency.
However, since such damping foil exists, because it is arranged in the flux path, generation of eddy currents cannot be avoided and the problem arises of fluctuations of potential on the damping foil being detected as noise, due to electrostatic coupling with the exciting coils. There were therefore, in addition, the drawbacks that the construction was complicated due to the need for anti-noise measures such as the requirement to provide measures such as arranging an insulating layer between the guard electrodes and the damping foil.
Also, as described above, since the output impedance from the face electrodes is extremely high, the input impedance of the amplifier needs to have a high value of the order of a few G xcexa9. Slight changes in the insulating characteristics of this portion produce errors of measurement, so the interior of the measurement tube around the face electrodes and guard electrodes was filled with epoxy resin, with the object of preventing any decrease in insulation.
However, if the method was adopted of fixing these components by packing with epoxy resin, stress was generated between the face electrodes and guard electrodes when this heated resin contracted, causing cracks, with the risk of decrease in insulation. Furthermore, since the face electrodes and guard electrodes were of large size, if mechanical vibration of the entire detection unit was produced by the fluid flowing through the interior during measurement, differences were produced in the output impedance of the two face electrodes, resulting in the production of induction noise. Also, friction noise was generated by the vibration of the signal cable.
Accordingly, one object of the present invention is to provide a novel, stable capacitative electromagnetic flow meter which is little affected by differentiation noise (electromagnetic induction noise) or electrostatic induction noise or friction noise and which is little affected by fluid noise and with excellent resistance to vibration and humidity, by reducing to the utmost problems generated by the diamagnetic effect in the flux path.
In order to achieve the above object, the present invention is constituted as follows. Specifically, according to the present invention,
a capacitative electromagnetic flow meter comprises:
a measurement tube made of insulating material through which flows a fluid to be measured;
an exciting coil wound on a magnetic pole arranged facing the periphery of the measurement tube, that supplies flux in a direction orthogonal to the tube axis direction of the measurement tube;
a pair of face electrodes arranged facing the periphery of the outer wall of the measurement tube in directions respectively orthogonal to the direction of the flux and the tube axis direction of the measurement tube;
guard electrodes arranged so as to cover the face electrodes from the periphery thereof, maintaining a prescribed separation with the face electrodes;
an exciting circuit that supplies exciting current of a frequency of at least the commercially available frequency to the exciting coil;
a pre-amplifier that amplifies the detection signal detected through the electrostatic capacitances between the fluid to be measured and the pair of face electrodes, respectively, and between these face electrodes and the respective guard electrodes;
a cable whereby the face electrodes and guard electrodes are connected with the pre-amplifier;
a signal processing unit that outputs the flow rate of the fluid to be measured from an output signal of the pre-amplifier;
a cylindrical yoke forming a magnetic return circuit of the exciting magnetic field arranged coaxially with the measurement tube and so as to cover the periphery of the exciting coil;
a coil fixing plate of non-magnetic material electrically connected and fixed to the cylindrical yoke, covering the exciting coil; and
earth rings provided at both ends of the measurement tube, whereby a metal pipe that is coaxially arranged with this cylindrical yoke and the cylindrical yoke are arranged and fixed symmetrically and electrically connected with respect to the axis connecting the centers of the pair of face electrodes and the tube axis of the measurement tube, at the periphery of the cylindrical yoke; wherein
the exciting circuit comprises filter means that controls the waveform of the exciting current such that the exciting flux waveform has a flat section; and the value of the electrostatic capacitance formed between the face electrodes and the guard electrodes is smaller than the value of the electrostatic capacitance between the fluid to be measured and the face electrodes.
Consequently, since, according to the present invention, the frequency of the exciting current is high and is controlled within a prescribed settling time, damping foil is unnecessary and eddy currents of the magnetic circuit are suppressed, thereby making it possible to arrange for the exciting flux waveform to have a flat section, so flow rate measurement can be achieved in a stable fashion with high accuracy without being subject to the effects of differentiation noise, electrostatic noise or fluid noise.
Also, since electrostatic induction noise from the exciting coils is screened by the coil fixing plate and the electrostatic capacitance between the face electrodes and the guard electrodes is small and the amplification gain of the induction noise superimposed on the detection signal is low, a capacitative electromagnetic flow meter can be obtained that is resistant to induction noise.